Fixing ERR_HTTP_HEADERS_SENT in Node.js and Express

The ERR_HTTP_HEADERS_SENT error occurs in Node.js and Express applications when an attempt is made to modify the HTTP headers of a response after they have already been sent to the client. This can happen if you’re trying to send a response to the client but the response has already been transmitted, or if a response is being sent multiple times.

Reasons Behind the Error

• Multiple Responses: Sending more than one response to a client request.
• Async Operations: Attempting to set headers after an asynchronous operation has completed and a previous response has already been sent.

Steps to Fix the Error

1. Be sure to only send one response per request.
2. Use return statements to prevent further code execution after sending a response.
3. Check your route handlers for any unintended multiple response calls.
4. Ensure you handle errors properly so that they don’t lead to additional responses being sent.

Complete Code Example

const express = require('express');
const app = express();

app.get('/', (req, res) => {
  // Imagine some logic here that may result in sending a response
  res.send('First response');

  // More logic here that attempts to send another response by mistake
  // This will cause ERR_HTTP_HEADERS_SENT
  // res.send('Second response'); // Comment this out to prevent the error
});

app.listen(3000, () => {
  console.log('Server is running on port 3000');
});

Additional Solutions

If your code involves asynchronous operations (like database queries), be sure to handle the response only once. For example:

app.get('/user', (req, res) => {
  User.findById(req.params.id, (err, user) => {
    if (err) {
      // Handle the error and send a response
      return res.status(500).send(err);
    }
    if (user) {
      // Send user data as a response
      res.send(user);
    } else {
      // Handle case when user isn't found
      res.status(404).send('User not found');
    }
    // No further response should be sent here!
  });
});

Every response method (like res.send, res.json, res.end) should be called once per request. Using return with these methods can help prevent further execution which might cause additional response attempts.